Little is known about the types of organisms that inhabited Earth before the Great Oxygenation Event (GEO) – also known as the Oxygen Crisis – which led to the extinction of most life forms around 2.5 billion years ago. Fortunately, researchers have now reported the discovery of fossilized bacteria that lived just prior to this pivotal moment in our planet’s history, providing new insights into what life was up to during this formative period.
Known as the Archean Eon, the era leading up to the GEO saw the emergence of photosynthesizing algae that filled the air with oxygen. Initially, most of this oxygen was captured by iron in rocks, which reacted with it to form iron oxides. However, after a while, these iron sinks became exhausted, meaning the oxygen produced by these algae was left to float freely in the atmosphere.
This free oxygen was toxic to the anaerobic microbes that made up the bulk of life at the time, and therefore caused an apocalyptic die-off. While the fossils of some Archean organisms have since been discovered, most of these are from shallow water environments, and our knowledge of deeper marine ecosystems during this period remains somewhat scant.
Yet the authors of a new study in the journal Geology describe the discovery of “large, organic, smooth-walled” microfossils, recovered from a rock bed called the Gamohaan Formation within an ancient piece of Earth’s crust known as the Kaapvaal craton in South Africa.
Dated back to 2.52 billion years ago, the fossils were surrounded by iron sulphide – or pyrite – containing a number of sulphur isotopes that suggest the organism was a sulphur-oxidizing bacteria, similar to those that live in deep-water volcanic vents today.
Existing in an environment lacking in oxygen yet high in sulphur, the bacteria probably played an important role in its local ecosystem, providing a source of sulphate for other microbes to use in their metabolic processes.
According to the researchers, this is the oldest example of a sulphur-oxidizing bacteria ever discovered, and provides the first evidence that this type of microbe was present prior to the Oxygen Crisis.